Systems that use liquids, such as propellants and coolants, usually require a means for determining the quantity of liquid remaining in a storage tank. For such systems operating on Earth, determining the quantity of liquid in the storage tank can easily be done because the liquid and ullage form a flat liquid/vapor interface. For example, the height of the liquid in the tank may be measured or the liquid pressure at the bottom of the tank may be gauged.
However, conventional gravity-based liquid measurement methods are not feasible in microgravity environments, such as encountered in Earth orbit or other space travel. Due to the lack of or low gravitational force in such environments, the liquid and ullage may have unknown or uncontrolled locations within the tank, possibly resulting in numerous vapor bubbles dispersed throughout the tank. Consequently, gauging the quantity of liquid in the tank cannot be readily carried out by conventional means.
Several methods are known for attempting to gauge liquid quantity in microgravity environments. One such method, commonly termed the mass-accounting method, requires knowledge of the initial liquid quantity and of all subsequent usage rates. The amount of liquid remaining is simply the difference between the initial quantity and the quantity estimated to have been removed. This method poses accuracy concerns because it does not account for leakage losses. Accuracy concerns also arise from tolerance buildup. That is, this method relies on prior measurements of initial mass and usage rates to infer the current quantity, rather than on direct measurements of the current quantity. The errors in the prior measurements, even if insignificant in themselves, add up to a significant error in the calculation of the current quantity.
Another method, called nucleonic or radiation attenuation gauging, involves placing a radiation source on one side of the tank and a Geiger counter on the other side of the tank. Liquid between the source and the Geiger counter attenuates the radiation. Therefore, the volume of the liquid in the tank can be inferred from the amount of radiation detected by the Geiger counter. However, this method creates safety hazards and side effects of radiation on the electronics. In addition, this method is impractical for use with large tanks or in a high background radiation environment.
Yet another method involves imparting an acceleration to the space vehicle, thereby settling the liquids and allowing the liquid level to be sensed. This method has several drawbacks. This method causes disturbances to the space vehicle and requires expenditure of significant amounts of propellants.
Other methods measure the change in pressure upon addition of either heat or gas to determine the volume of the liquid in the tank. These methods also have drawbacks. The determination of the quantity of liquid is subject to significant errors if the liquid is not isothermal. Isothermal conditions are unlikely in cryogen tanks, especially during expulsion or fill, unless the contents of the tank are actively mixed. Furthermore, since these methods require the injection of noncondensible gas or heat each time a pressure reading is taken, there is a limit to the number of measurements that can be taken.
Accordingly, it is an object of this invention to provide a method and system for determining liquid quantity in a tank in microgravity in which tolerance buildup and leakage losses will not adversely affect the accuracy of the determination.
It is another object of this invention to provide a method and system for determining liquid quantity in a tank in microgravity which eliminates radiation concerns.
It is yet another object of this invention to provide a method and system for determining liquid quantity in a tank in microgravity which does not require the use of a settling thrust on the spacecraft.
It is yet another object of this invention to provide a method and system for determining liquid quantity in a tank in microgravity which does not require injecting gas into the tank each time a determination of liquid quantity is made.
It is yet another object of this invention to provide a method and system for determining liquid quantity in a tank in microgravity which does not limit application to isothermal fluids.